Valves manufactured by all suppliers are sticking due to a chemical reaction to a rust inhibitor that was used on one part inside scroll compressors. Most of the time these valves will underfeed. A system with a valve that is clogged with goo will operate with a low suction pressure, a somewhat low high side pressure, and a high superheat. It can look like an undercharge. However, an undercharged system will have low or no subcooling while a properly charged system with a clogged valve will have a normal to high subcooling. The subcooling is really the key to telling the difference between an undercharged system and an underfeeding valve. If you suspect a clogged valve, be sure to check for other refrigerant restrictions – such as a clogged filter drier. Of course if you change the valve, you will also want to change the filter.
A valve that has lost its bulb charge will also underfeed, but these normally are drastic – with the low side pulling down close to 0 psig. Valves can lose their charge from improper installation. If the bulb is attached to the suction line near where the suction line is brazed in and you don’t protect it from heat, the bulb pressure can pop the bulb charge.
Overfeeding valves are normally due to misapplication or poor installation. A system with an overfeeding valve will have a high suction pressure, a low superheat, and a low subcooling. If you see this, check the bulb installation. Make sure it is making good contact with the suction line and is well insulated. Also, make sure the external equalizer comes off the top of the suction line, not the bottom.
Unfortunately it is no longer true that expansion valves rarely mess up. However, checking the valve installation and system subcooling may save you a bunch of time and trouble. It takes far less time to check system subcooling, superheat, and the valve installation than it does to change a valve.